The UCCRC Transgenic Mouse/Embryonic Stem Cell Facility provides comprehensive genetic engineering services to alter the genome of the laboratory mouse. The UCCRC has many investigators who use mouse models as their primary tool of analysis, as well as a multitude of additional investigators who require the occasional mouse model in their studies of the causes and treatment of cancer. The generation and maintenance of transgenic animals through the microinjection of singlecelled mouse embryos, and the generation of genetically modified mice through the use of ES cells require specialized technical personnel. Furthermore, such efforts necessitate the acquisition of an array of devoted equipment. Thus, the availability of a shared facility greatly reduces research costs for the individual investigators of the UCCRC. The existence of this facility also greatly increases the accessibility of genetic engineering technology to investigators with limited related experience. The UCCRC Transgenic Mouse/Embryonic Stem Cell Facility was established in 1991, and has been tremendously productive and successful;the range of services provided has expanded considerably since its founding. Nine services are provided by the Transgenic Mouse/Embryonic Stem Cell Facility: (1) transgenic mouse production from founder through F1 Stage;(2) ES cell technology mouse production;(3) ES cell gene targeting and culturing;(4) mouse embryonic feeder (MEF) cell production; (5) Timed pregnancies of various strains and lines of mice;(6) embryo rederivation;(7) ES cell line development;(8) various breeding services and GEM model line maintenance;and (9) DMA preparation from ES cell lines for the construction of a gene targeting construct. In addition to the technical services, the Facility also offers assistance with the design of studies that require mouse molecular genetics and an annual course in Mouse Handling and Breeding. This course covers information regarding analysis and handling of the mice once the Facility has generated them, as well as general breeding and handling procedures for mice. In providing these comprehensive services, the UCCRC Transgenic Mouse/Embryonic Stem Cell Facility has generated mouse models that have led to advances in our understanding of cancer, including cancers of the breast, skin, colon, brain and hematopoietic system.